The present invention, in some embodiments thereof, relates to novel compounds, methods of preparing same, metal complexes formed therewith and uses thereof and, more particularly, but not exclusively, to novel dialkylphosphine-containing compounds and diarylphosphine-containing compounds that can be utilized for forming a library of tridentate ligands, such as, for example, pincer-type ligands, to methods of synthesizing these compounds and ligands, to uses thereof in, for example, the preparation of organometallic complexes and to the various uses of such organometallic complexes.
Organophosphorus compounds have enjoyed a variety of important applications in numerous actively developing fields of science and technology. Organophosphorus compounds have found use as agricultural insecticides, anti-corrosion and fire-resistant agents, extractants in hydrometallurgy and as antimicrobial and chemotherapeutic agents.
In addition, phosphorus-containing compounds have received special attention due to their spectacular applications in synthetic chemistry both as reagents and ligands for metal-based catalysis. From a synthetic point of view, phosphorus-containing precursors decorated with versatile functional groups are especially valuable, particularly if these groups can be easily interconverted and further diversified.
One of the most interesting synthetic applications of organophosphorus compounds is the formation of tridentate pincer type ligands, also referred to herein and in the art as “pincer ligands”.
A pincer ligand is a type of a chelating agent that can bind tightly to three adjacent coplanar sites, usually on a transition metal. Typical tridentate pincer type ligands have the general form D1CD2, wherein C is a carbon atom that can potentially interact with a metal; and D1 and D2 are groups containing coordinating atoms (also referred to herein as electron donating atoms). In most pincer ligands, the carbon atom forms a part of an aryl ring, typically phenyl. The carbon atom can be replaced by other coordinating atoms such as nitrogen or sulfur, which typically form a part of a heterocyclyl such as a heteroaryl.
Many useful pincer ligands contain phosphines. Early examples of pincer ligands are anionic ligands with a carbanion as the central donor site and flanking phosphine donors. Such ligands are referred to in the art as PCP pincers. Other pincer ligands include, for example, PNP, SCS, NCN, PCS, PCN, PNN, and NNN.
The assumed irreversibility of pincer-metal interaction confers high thermal stability to the resulting complexes. This stability is ascribed to the constrained geometry of the pincer ligand and steric shielding provided to the metal center by substituents of the coordinated donor groups.
Stoichiometric and catalytic applications of pincer complexes have been studied at an accelerating pace since the mid 1970's, especially for C—H activation.
Tridentate pincer type ligands have found spectacular employment in coordination, mechanistic, synthetic and supramolecular chemistry, along with nanoscience and the development of sensors and molecular switches. Most significantly, a realization that pincer ligands offer both a unique, highly protective environment for the coordinated metal center and opportunities to fine tune the steric and electronic metal properties has generated extensive research into the use of these complexes as catalysts [For reviews, see: J. T. Singleton, Tetrahedron 2003, 59, 1837; and D. Morales-Morales, Rev. Quim. Mex. 2004, 48, 338]. As a result, many important and challenging catalytic processes have been developed based on such systems.
It is generally accepted that the reactivity, selectivity and catalytic performance of pincer-based systems greatly relies on the characteristics of the donor groups D in the carefully selected ligand. These characteristics depend on the type of the coordinating atom, and further on the nature of its organic substituents.
Catalytic groundbreaking processes such as dehydrogenation of saturated hydrocarbons and dehydrogenative coupling of alcohols with amines [Gunanathan, C.; Ben-David, Y.; Milstein, D. Science 2007, 317, 790] have been developed on the basis of pincers bearing bulky electron-donating phosphorus substituents.
Pincer-type ligands based on bulky, electron-donating phosphines are therefore advantageous for numerous important applications.
Traditionally, pincer ligands are prepared by attaching electron donating atoms, or groups containing the same, to a ligand backbone.
Optimization of tailor-made catalysts therefore includes extensive experimental investigations, in which laborious ligand synthesis is often a serious bottleneck. Especially, synthesis of non-symmetrically substituted D1CD2 ligands (D1 and D2 are different groups) represents a considerable challenge, as their preparation usually includes series of steps and separations which commonly result in low yields.
The preparation of mono-phosphine and chiral phosphine ligand libraries, using the “click” reaction has recently been reported [Q. Dai, W. Gao, D. Liu, L. M. Kapes, X. Zhang, J. Org. Chem. 2006, 71, 3928; R. D. Detz, S. Heras, R. de Gelder, P. W. N. M. van Leeuwen, H. Hiemstra, J. N. H. Reek, J. H. van Maarseveen Org. Lett. 2006, 8, 3227; F. Dolhem, M. J. Johansson, T. Antonsson, N. Kann, J. Comb. Chem. 2007, 9, 477.
The “click” reaction is a name used to describe a Cu(I)-catalyzed stepwise variant of the Huisgen 1,3-dipolar cycloaddition of azides and alkynes to yield 1,2,3-triazole. This reaction is carried out under ambient conditions and with exclusive regioselectivity for the 1,4-disubstituted triazole product when mediated by catalytic amounts of Cu(I) salts [V. Rostovtsev, L. G. Green, V. V. Fokin, K. B. Sharpless, Angew. Chem. Int. Ed. 2002, 41, 2596; H. C. Kolb, M. Finn, K. B. Sharpless, Angew Chem., Int. Ed. 2001, 40, 2004.
Pincer ligands based on alkyl-substituted phosphines have recently demonstrated broad use in the elucidation of elusive species [For representative examples, see: (a) van Koten, G.; Timmer, J. G.; Noltes, J. G.; Spek, A. L. J. Chem. Soc., Chem. Commun. 1978, 250. (b) Albrecht, M; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 2001, 123, 7233. (c) Gandelman, M.; Rybtchinski, B.; Ashkenazi, N.; Gauvin, R. M.; Milstein, D. J. Am. Chem. Soc. 2001, 123, 5372. (d) Vigalok, A.; Milstein, D. Acc. Chem. Res. 2001, 34, 798. (e) Poverenov, E.; Efremenko, I.; Frenkel, A.; Ben-David, Y.; Shimon, L. J. W.; Leitus, G.; Konstantinovski, L.; Martin, J. M. L.; Milstein, D. Nature 2008, 455, 1093.].
While a myriad of di-substituted phosphine ligands have been described in the art, the synthesis of dialkyl-substituted phosphine compounds such as, for example, dialkyl substituted propargyl, azidomethyl, bromomethyl, and carboxymethyl phosphine species, has not been described hitherto, with the exception of dialkylphosphinyl acetic acids [Dolhem, F; Johansson, M. J.; Antonsson, T.; Kann, N. Synlett 2006, 20, 3389].